Investigation On The Oxygen Permeation Performance Of La_1-xSr_xCo_1-yFe_yO_3-δ Hollow Fiber Membranes Modulated By (La_0.5Sr_0.5)₂CoO_4+δ

The powders of perovskite oxide La_1-xSr_xCo_1-yFe_yO_3-δ were successfully prepared by the sol-gel method, and gas-tight LSCF hollow fiber membranes were fabricated through the phase inversion-sintering process. The K2NiF4-type oxide, （La0.5Sr0.5）2CoO4+δ （LSC214）, was used as the surface decoration surface catalyst to improve the oxygen reduction reaction in La_1-xSr_xCo_1-yFe_yO_3-δ membranes through reducing the apparent activation energies for enhancing oxygen permeability. Moreover, the effect of micro-structure of hollow fiber membranes on improving the bulk diffusion process of oxygen ions was investigated by modulating the micro-structure, then reducing the apparent activation energies, to achieve the improvement of oxygen permeability.（1） La0.6Sr0.4Co0.2Fe0.803-δ（LSCF）,the normally used material, was chosen as the substrate membrane, and acid etching （increase the roughness） followed by dispersed-second-phase particle decoration, a combinatorial method promises to improve the surface modification effect synergistically, was employed here. It was found that surface treatment with LSC214 indeed contributes to the improvement of LSCF on oxygen permeability, and much more effectively than acid etching progress. Compared to the 0.036-1.021 ml·min-1·cm-2 of bare membrane, the oxygen permeation fluxes of surface modified LSCF membranes were dramatically enhanced to be 0.201-1.311 ml·min-1·cm-2 of the LSC214-decorated membrane, with enhancement factor of 4.93, when temperature rose from 700 to 1000℃ （helium flow rate:150 ml·min-1）. The modification of LSC214 on LSCF greatly reduced apparent activation energy with 43～57% to improve the oxygen permeability. In addition, the surface decorated membrane also exhibited high oxygen permeation stability during the entire operating 200 hours at 900℃ without flux decay （T= 900℃, flow rate of sweep gas=100 ml·min-1, air flow rate= 200 ml·min-1）.（2） La0.7Sr0.3FeO3-δ（LSF） was chosen as the substrate membrane, gas-tight LSF hollow fiber membranes with different micro-structures（LSF-a、LSF-b） were prepared by controlling the internal coagulants to probe the resistance of oxygen permeation process, following with LSC214 modification. The undecorated LSF-a and LSF-b membranes have oxygen permeation fluxes of 0.008-0.348 and 0.246-5.233 ml·min-1·cm-2, respectively, in the temperature range of 700 to 1000℃ and the flow rate of helium gas at 150 ml·min-1. Through the modulation on micro-structure, LSF-b hollow fiber membrane possesses excellent oxygen permeation fluxes, which are about 16 times of those of LSF-a membrane. The surface decoration greatly decreased the apparent activation energy of oxygen transport. The apparent activation energy of LSC214-decorated LSF-a （82.67 kJ·mol-1） was less than that of the bare LSF-a （138.55 kJ·mol-1）, similarly, that of the coated LSF-b （44.54 kJ·mol-1） was also smaller than the bare LSF-b （95.88 kJ·mol-1）. After the LSC214 modification, it can be seen that the maximal oxygen permeation flux of LSF-b hollow fiber membrane was about 7.199 ml·min-1·cm-2, which was almost 19 times of that of the bare LSF-a membrane at the same conditions （flow rate of helium:300 ml·min-1; temperature:1000℃）.（3） La1-xSrxCoO3-δ（x=0.2,0.4） was chosen as the substrate membrane followed with LSC214 modification conduct. La0.6Sr0.4CoO3-δ（LSC64） hollow fiber membrane with different micro-structures（LSC-a、LSC-b） were prepared by controlling the viscosity of the spinning mixture to probe the resistance of oxygen permeation process. The undecorated LSC-a and LSC-b membranes have oxygen permeation fluxes of 0.006-0.419 and 0.279- 1.525 ml·min-1·cm-2 respectively. In the temperature range of 750 to 950℃ and the flow rate of helium gas at 100 ml·min-1. LSC-b membranes exhibited high oxygen permeation stability at 950℃ without decaying, about 1.5 ml·mi-1·^cm-2 and 2.8 ml·min-1·cm-2 for the undecorated and LSC214-decorated membranes, respectively. The surface decoration greatly decreased the apparent activation energy of oxygen transport. The apparent activation energy of LSC214-decorated LSC-a （90.00 kJ·mol-1） was less than that of the bare LSC-a （224.10 kJ·mol-1）. Similarly, that of the coated LSC-b （30.93 kJ·mol-1） was also smaller than the bare LSC-b （90.15 kJ·mol-1）. Similarly, the apparent activation energy of LSC82 hollow fiber membrane decreased from 126.45 kJ·mol-1 to114.13 kJ·mol-1. All these results demonstrated that the modification treatment by LSC214 could improve the oxygen permeability of La1-xSrxCoO3-δ system.（4） spin-polarized density functional theory calculations employing the generalized gradient approximation（GGA）+U scheme were used to investigate the atomic, electronic structures of La1-xSrxCoO3 （x=0.0,0.125） materials. Nonstoichiometric symmetrical slab models were adopted. The thermodynamic stability of low-index surfaces was analyzed with phase diagrams drawn with the total energies obtained from calculations. The influence of Sr-doping on the thermodynamic stability of the surfaces was also examined. In undoped LaCoO3 system, CoO2-and LaO-terminated （001） surfaces are the most stable two of all considered surfaces under typical operational conditions of the oxygen permeation material（T= 1100 K, pO2= 0.2 atm）, while Sr-doping in LaCoO3 crystal destabilizes the CoO2-terminated surface with respect to the Lao.75Sro.250-terminated surface.